The present invention relates generally to the remediation of contaminated soil.
In particular, the invention relates to a method of remediating soil that has been contaminated with heavy metals.
Contamination of the soil with heavy metals has long been a major environmental concern. Contamination from heavy metals, especially cadmium, lead and mercury, may be caused by such industrial activities as metal-processing, tanning, chemical processes employing metal catalysts, etc.
There have been several proposals directed to solve the problem, but none that has proved entirely satisfactory.
One prior method consisted of treating the soil in situ with solutions of alkali sulfides, and percolating the solutions through the soil to cause the heavy metal cations to react with the sulfide anions and yield very low-soluble sulfides. (For example, the solubility products of cadmium, lead and mercury are 1.4xc3x9710xe2x88x9228, 1.0xc3x9710xe2x88x9229, and 3.0xc3x9710xe2x88x9253, respectively.)
The cations of the heavy metals are blocked, by virtue of the above compounds being insoluble, and are no longer in a condition to contaminate springs and crops.
With such a method, however, a conversion rate into insoluble sulfides of no more than 70% is obtained, even where the soil comprises a substantial proportion of sand, making for better contact of the alkali sulfide solution with the heavy metal compounds.
In an attempt at improving the above method, it has been proposed (DE 19547271) of treating the soil with an acid solution, specifically a hydrochloric acid solution, subsequently to the step of percolating the soil with the sulfide solution. In this way, a conversion rate of heavy metals to sulfides upward of 99% is reportedly obtained.
However, the last-mentioned method has a major limitation in that it is only successful where the soil mostly comprises sand, since in this case good contact can be ensured between the reactant (alkali sulfide) and the heavy metal cations. On the other hand, a soil that is rich in clay or other cohesive components would hinder that contact, and the conversion to insoluble sulfides becomes incomplete.
The problem underlying this invention is to provide a method of remediating soil that contains heavy metals, whereby the aforementioned deficiencies of prior methods can be overcome.
The problem is solved, according to the invention, by a method comprising the steps of:
removing and sieving a heavy metal-containing soil to remove stones and gravel; and
treating said sieved soil, arranged in a thin layer and maintained in a highly turbulent condition, with a solution of an alkali sulfide at a temperature of at least 50xc2x0 C.
Preferably, the step of treating the sieved soil with an alkali sulfide solution is preceded by a step of adjusting the soil pH to a value equal to or lower than 6.
The method is implemented more advantageously in an apparatus known as a xe2x80x9cturbo-reactorxe2x80x9d.
In this case, the inventive method comprises the steps of:
removing and sieving a heavy metal-containing soil to remove stones and gravel;
feeding a continuous stream of said soil into a turbo-reactor, which reactor comprises a cylindrical tubular body being laid with its axis horizontal, closed by end walls at its opposite ends, and provided with inlet openings for the soil to be treated and for at least one reactant, as well as provided with at least one discharge opening, a bladed rotor rotatably mounted in the cylindrical tubular body and driven at a high rotational speed to produce a stream of finely divided soil particles, and a heating jacket for raising the temperature of the inner wall of the cylindrical tubular body to at least 110xc2x0 C.;
feeding a continuous stream of a reactant in the form of an aqueous solution of an alkali sulfide into the turbo-reactor in cocurrent with the soil stream; centrifuging the soil particles and the alkali sulfide solution against the inner wall of the cylindrical tubular body to form a highly turbulent, tubular dynamic fluid layer wherein the soil particles and the alkali sulfide solution are urged mechanically in intimate mutual contact by the rotor blades; and
reacting the soil and the alkali sulfide in the thin layer while the latter is being urged, substantially in contact with the heated inner wall, toward said at least one discharge opening of the turbo-reactor, with simultaneous generation of steam.
Said step of feeding in a continuous stream of an aqueous solution of alkali sulfide is preferably preceded by a step of adjusting the soil pH to a value equal to or lower than 6.
This pH adjusting step is carried out conveniently by feeding into the turbo-reactor a continuous stream of an aqueous acid solution in cocurrent with the soil stream.
An aqueous solution of a strong acid selected from hydrochloric acid or sulfuric acid is preferred, at a concentration in the 0.01N to 1N range, advantageously equal to 0.1N.
An opening for exhausting any vapors released during the treatment may be provided conveniently, and the exhaust opening may be connected to a scrubber for removing any hydrogen sulfide formed when the soil is markedly acidic.
To enhance the heavy metal insolubilization process, a continuous stream of an alkali silicate, having complexating and agglomerating properties, may be fed into the turbo-reactor through an inlet opening provided downstream of the inlet opening for the alkali sulfide solution.
The alkali sulfide solution is preferably a sodium sulfide solution, with a concentration of 5 to 15%, preferably about 12%, w/v.
The temperature of the turbo-reactor inner wall is preferably 110xc2x0 to 220xc2x0 C.
The treated soil exits the turbo-reactor at a temperature of about 50xc2x0 to 90xc2x0 C.
The peripheral velocity of the bladed rotor is preferably 20 to 40 meters per second.
The average time of residence of the soil being processed in the turbo-reactor varies generally between 30 and 120 seconds.
The amount of alkali sulfide solution used in the method of this invention generally exceeds the stoichiometric amount demanded by the proportion of heavy metals in the soil, as evaluated by a preliminary analysis of the soil composition. This because other metals, such as iron, present in the soil would also react with the sulfide.
The application of the inventive method results in a practically quantitative formation of insoluble sulfides from the cations of heavy metals in the soil, irrespective of the soil characteristics and its content in clay or cohesive components.
This is achieved by the creation of the aforementioned turbulent thin dynamic layer, in which the soil is divided into very fine particles so that heavy metal cations become liable to an intimate contact with the reactant. The reaction by which the cations are converted into alkali sulfides is promoted and accelerated by the elevated temperature of the turbo-reactor inner wall, the thin dynamic layer comprised of soil particles and tiny droplets of the reactant solution being forced to flow along said inner wall.
The method of this invention will be described in greater detail with reference to the accompanying drawing and through some exemplary and non-limiting embodiments thereof.